1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine, and more specifically to an air intake side secondary air supply system in which the accuracy of its operation when the amount of the secondary air is large is improved.
2. Description of Background Information
Air/fuel ratio feedback control systems for an internal combustion engine are known in which the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and the air/fuel ratio of mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and improvements of the fuel economy. As an example of the air/fuel ratio feedback control system, an air-intake side secondary air supply system of a duty ratio control type is proposed, for example, in Japanese Patent Publication No. 55-3533 in which an open-close valve is disposed in an air intake side secondary air supply passage leading to a part of an intake manifold, downstream of a throttle valve of a carburetor, and a duty ratio of the open and close condition of the open-close valve, i.e. the supply of the air intake side secondary air, is feedback controlled in response to the output signal level of the O.sub.2 sensor.
In such an air-intake side secondary air supply system of the duty ratio control type, the amount of the secondary air flowing through the open-close valve is saturated and changes very little with respect to the change in the control signal in a large range (90.about.100%, for example) of the duty ratio which indicates a time proportion of the opening of the open-close valve in each of the duty periods. Therefore, under such a condition, the amount of the secondary air does not necessarily correspond to the duty ratio of the control signal. Thus, with the conventional systems, the accuracy of the air/fuel ratio control may not be maintained when the duty ratio of the control signal is relatively large.
On the other hand, there is an air intake side secondary system in which a linear type solenoid valve is provided in the air intake side secondary air supply passage leading to the intake manifold. Such an air intake side secondary air supply system is disclosed, for example, in Japanese Patent Application laid-open No. 55-119941. In this system, an opening degree of the linear type solenoid valve is varies in response to the magnitude of a drive current supplied to its solenoid. With this solenoid valve, a sectional area of the air intake side secondary air supply passage is varied in response to a result of detection of the oxygen concentration in the exhaust gas.
In this type of air intake side secondary air supply system, the opening degree of the linear type solenoid valve does not vary precisely in proportion to the value of the drive current. Especially, in a range where the magnitude of the supplied current is large, the change in the opening degree of the solenoid valve per unit current value becomes small. Therefore, with conventional air/fuel ratio control systems of this type, the amount of the secondary air may deviate from the proper value, to reduce the accuracy of the air/fuel ratio control when the magnitude of the drive current to the linear type solenoid valve is large.